1. Field of the Invention
The present invention relates to a transmitter-receiver for automatically establishing radio communications with other transmitter-receivers. The transmitter-receiver includes a preset selective call receiver, an adjustable selective call generator, and an oscillator adjustable to several channels, wherein, in the ready-to-receive condition, the channels are cyclically and sequentially connected until a call is determined, and wherein, for establishing a connection with another station, the channels are successively connected until an idle channel is found.
2. Description of the Prior Art
Such transmitter-receiver units are known (German Published Application 2,139,403). They are used in radio systems where, for optimum utilization of the radio channels in a frequency band, each user has access to each "idle" channel through an automatic channel finding circuit. In star-type radio networks, i.e. where traffic is handled via a central station, idle channels are identified by a special modulation. In meshed radio networks, i.e. with direct traffic between the users, identification of idle channels is impossible because there is no central station. If a station is ready to receive, its receiver cyclically searches all channels in the frequency band for signals which are intended for this station.
If, however, a party at one station wants to call another station, a preliminary check must be performed to find out which of the channels is idle. A channel is idle if the RF energy received on this channel does not exceed a predetermined threshold value. This means that the channel is not occupied by other stations of its own network nor by outside transmitters nor by interfering RF energy sources.
Under these latter conditions it can be expected that the called station will be received interference-free with a high probability. Since the location of the other station and, consequently, the radio-link losses are unknown, the receiver is operated with its maximum sensitivity. Therefore, the threshold above which the RF energy must lie in order that a channel can be considered "idle" is only slightly above the equivalent noise input of the receiver.
In the known transmitter-receiver units, "idle" testing is done by measuring the received field strength. The components needed for that purpose make such a receiver very expensive.